If a disk in a disk set on Mac fails or is missing
If you see a message that a disk is missing or has failed, there are several things you can try to solve the problem.
If a disk is missing: Make sure it’s connected to power, turned on, and connected to your computer. If you are using a FireWire or USB disk, you may need to disconnect and reconnect it.
If you see a message that a disk is missing or has failed: Use the First Aid pane to repair the RAID set. See Repair a disk.
If you think it’s a problem with your Mac or Disk Utility: Quit and reopen Disk Utility or restart your Mac, then open Disk Utility again and check the RAID set.
If you are using a mirrored RAID set: Disconnect the disk and then reconnect it. Open Disk Utility and repair the disk. See Repair a disk in a mirrored disk set.
If you are using a striped RAID set: Delete the damaged RAID set. Your data may be lost. Be sure to back up the disks in your RAID set and other data regularly. See Delete a disk set.
If you have a persistent problem with a disk in a mirrored RAID set: You may need to replace it. After you replace the damaged disk, you need to add the new disk as a disk set member and rebuild it. See Repair a disk in a mirrored disk set.